Colorectal cancer is particulary refractory to chemotherapeutic approaches. While 5-fluorouracil (FUra) is the single agent of choice in this disease state, only 15-20% of patients respond to this treatment. Recent clinical studies have reported significant increases in response to the combination of FUra and folinic acid (CF). CF is thought to direct the action of FUra to inhibition of thymidylate synthase (TS), an enzyme pivotal in DNA biosynthesis and critical for tumor cell growth. The potential that the FUra- CF combination will become an established protocol for the therapy of colorectal carcinoma provides the rationale for studies of factors governing response to this combination. The diversity of tumor phenotypes encountered clinically has limited the utility of in vitro chemotherapeutic sensitivity analyses to the in vivo setting. For this reason, a panel of human colorectal carcinoma cell lines of widely differing phenotype is being utilized to assess the response to fluoropyrimidine (FP)-CF combinations. Since TS is thought to be the target of FP-CF, these cells have been characterized with regard to biochemical and molecular parameters related to TS. It is the aim of this proposal to utilize these cell lines as a model system to elucidate the extent of variation among the cell lines in FP response modulation by CF and to identify the mechanism(s) that underlie variation in response to CF modulation of FP cytotoxicity. Ultimately, these studies may lead to enhancement of the therapeutic action of FP-CF combinations and to the identification of pre-therapeutic markers of response to such combinations.